Organic electroluminescent elements (hereinafter abbreviated as “organic EL elements”), which emit light from a luminescent layer provided between two electrodes in response to voltage applied to the electrodes, have been intensively studied and developed for various light sources, such as flat-panel illumination devices, light sources for optical fibers, backlights for liquid crystal displays and liquid crystal projectors, and other display devices. The recent interest especially in the above-mentioned industrial fields has been particularly focused on the organic EL elements, which are advantageous to high luminous efficiency, low voltage driving, light weight, and low costs.
The organic EL elements allow electrons to be injected from the cathode and holes to be injected from the anode and recombine the electrons and holes at the luminescent layer to generate visible light corresponding to the luminescent properties of the luminescent layer.
A typical anode is composed of indium tin oxide (hereinafter abbreviated as “ITO”), which has the highest electrical conductivity, a higher work function, and a higher efficiency of hole injection among transparent conductive materials.
A typical cathode is a metal electrode, which is typically composed of Mg, Mg/Ag, Mg/In, Al, or Li/Al, to ensure the efficiency of electron injection and the work function.
These metals have a high light reflectance, and can reflect the light from the luminescent layer to increase the intensity of emitted light (luminance), in addition to the function as an electrode (cathode). That is, the light directed toward the cathode is specularly reflected by the surface of the metal cathode and emerges from the transparent ITO electrode (anode).
Unfortunately, the organic EL element including such a specular cathode having a high light reflectance significantly reflects external light in a non-luminescent mode.
In other words, the display is disturbed by external light such as interior illumination and cannot express dark colors at bright sites. The organic EL element has too low contrast at bright sites to be used as a light source for a display device.
To prevent the reflection of external light, for example, Japanese Unexamined Patent Application Publication No. H8-321381 discloses a technique using a circular polarizer (also referred to as “circularly polarizing plate”). A typical circularly polarizing plate includes a protective film, a polarizer, and a λ/4 retardation film, in sequence from a viewer side.
The λ/4 retardation film for the circularly polarizing plate, which shows a higher retardation than a typical retardation film for a large liquid crystal display device, must be stretched at a higher rate to have a smaller thickness and achieve a retardation λ/4. Unfortunately, the studies of the present inventors revealed that the λ/4 retardation film fabricated by the high-rate stretching is readily affected by its storage conditions such as temperature and humidity and the organic solvents. For example, the λ/4 retardation film after the storage in a high-temperature high-humidity environment shows a large variation in size.
The circularly polarizing plate including such a λ/4 retardation film significantly curves due to the large difference in size variation rate between the protective film and the λ/4 retardation film, especially after the storage under conditions readily causing a size variation. This phenomenon leads to warpage of the plate and unavoidable reflection of external light. The curve of the circularly polarizing plates is not problematic in a conventional liquid crystal display device, which includes two polarizing plates disposed on both sides of a liquid crystal cell and mutually cancelling the effects of the curves.
In contrast, in the organic electroluminescent display device including only a single circularly polarizing plate on a viewer side, the curve of the circularly polarizing plate significantly affects the warpage of the organic electroluminescent display device. The repeated use under large variations in the temperature and humidity significantly deteriorates the organic electroluminescent display device.
Some conventional techniques are disclosed against the problems. For example, an elliptical polarizer is disclosed which includes a polarized-light separating film laminated on a quarter wavelength plate via an adhesive layer showing an excellent stress relaxation (refer to Patent Documents 1 and 2, for example). The adhesive layer is composed mainly of an acrylic polymer and has a relaxation modulus of 0.2 to 10 MPa. The adhesive layer prevents the photoelastic relaxation caused by the stress relaxation due to the internal heat of the laminate, to reduce the reflection loss of light and increase the use efficiency of light.
The present inventors checked for the visibility of the organic electroluminescent display device or the circularly polarizing plate having the configuration disclosed in Patent Documents 1 and 2 after the storage in a high-temperature high-humidity environment, and found that the shrinkage of both the λ/4 retardation film and the protective film in the circularly polarizing plate significantly affects the reflection of external light. This phenomenon is probably caused by the adhesive layer for the stress relaxation, which can prevent transmission of the stress from the shrinking layer to the λ/4 retardation film, but cannot sufficiently prevent a variation in size of the λ/4 retardation film itself due to variations in the temperature and humidity.
Another technique uses an elliptically polarizing plate, which includes a linearly polarizing plate, a first adhesive layer, a retardation plate, a second adhesive layer, an optical compensation plate (liquid crystal layer), and a third adhesive layer in this sequence, and has a storage elastic modulus of the second adhesive layer greater than that of the third adhesive layer (e.g., refer to Patent Document 3). The technique in Patent Document 3 can provide an elliptically polarizing plate having a small thickness and reducing cracks in a thermal shock test and creases due to the shrinkage of the linearly polarizing plate. Patent Document 3, however, does not disclose or suggest the stability of color tone or the visibility in the organic electroluminescent display device or the circularly polarizing plate including the λ/4 retardation film after the storage in a high-temperature high-humidity environment, although this problem is peculiar to circularly polarizing plates including λ/4 retardation films.
The organic electroluminescent element is readily deteriorated by uv rays. It is therefore desirable that any layer from the protective film of the circularly polarizing plate to the surface of the organic electroluminescent element have high uv absorptivity.
Unfortunately, if the protective film contains an increased amount of a uv absorbing agent to achieve high uv absorptivity, for example, the agent increases the haze and bleeds out. In addition, during application of a surface coating on such a protective film, the excess uv absorbing agent may be eluted from the protective film.
Most of the uv absorbing agents are immiscible with a polycarbonate resin and a cyclo olefin resin, which are typical components in the existing λ/4 retardation films. The uv absorbing agent therefore bleeds out and deteriorates the optical characteristics of the λ/4 retardation film.
To solve these problems, eagerly anticipated is the development of a circularly polarizing plate including a λ/4 retardation film fabricated by high-rate stretching, and an organic electroluminescent display device including the circularly polarizing plate, which are excellent in the stability of color tone, the stability of size, the resistance to the warpage due to the curved film, and the prevention of the reflection of external light, especially after the long-term storage in a high-temperature high-humidity environment.